1. Field of the Invention
The present invention relates in general to digital tape recorders employing a timer or metering roller to determine the amount of tape taken off a supply reel and, particularly, to preventing tape dropouts in such tape recorders by providing a specialized tape path.
2. Description of the Prior Art
Tape dropouts, that is, dropouts or interruptions in the signals recorded on, or reproduced from a magnetic tape, present a problem in all magnetic tape recording systems, however, this problem is particularly critical in digital tape recording, for example, as might be accomplished using pulse code modulation (PCM). Such dropouts have a particularly adverse influence on the signals being digitally recorded or reproduced because the wavelengths of the digital signals are much shorter than the wavelengths of the signals present when making analog recordings. Accordingly, a dropout occurring in a digital tape recording will result in missing a larger portion of the recorded information. Thus, the elimination of tape dropouts is much more critical in digital tape recorder design than in analog recorders, and efforts are continuously made in order to eliminate such tape dropouts.
Some known causes of tape dropouts are: irregularities in the thickness of the magnetic layer that is coated onto the tape base; dirt, dust, or other foreign particles that adhere to the surface of the tape; and scratches on the tape surface. Scratches are frequently made on the tape surface by the various rollers, spindles, and tape guides of the tape recorder when foreign matter adheres to their surface, or if there is a surface defect on the roller, either a pit or a raised particle. Not only do the surfaces of the rollers accumulate dust and foreign matter but so does the tape itself along the length of the tape which is exposed over the tape threading path between reels.
Therefore, in order to avoid dropouts in magnetic tape recorders, it is generally desirable to minimize the length of the exposed tape in the path between the reels and also to minimize the areas of contact of the tape with guide rollers and the like.
Another problem associated with the tape used in modern tape recorders is that it is necessary to employ a very thin magnetic layer on the tape base, in order to prevent self-magnetic reduction caused by mutual influence of the signals recorded adjacent each other. Moreover, in general, the base layer of the magnetic recording tape is typically made quite thin in order to provide a longer elapsed playing time at standard speeds, by permitting more tape to be wound on a conventional size reel. The base layer provides most of the mechanical properties of the tape and, thus, a very thin base results in an extremely flexible tape, which is necessary so that the tape can bend along the curves of the guide rollers and heads with good contact. Thus, the stress on the tape should be minimized to avoid damage to the thin tape, and the arrangement of the guide rollers and heads in the tape recorder is known to be a critical factor in determining the stress on the magnetic tape.
While it is understood that the arrangement of the tape guide rollers and heads must be taken into consideration, there are other constraints placed on the system design. For example, digital tape recorders typically employ a tachometer, driven by a roller in contact with the tape, to measure the running length of the tape, and the roller used with the tachometer is usually referred to as a timer roller. Because it is necessary to minimize slippage between the timer roller and the tape, so that there is no error made when measuring the length payed out, the wrap angle of the tape around the timer roller must be greater than around a conventional idler roller. This requirement in relation to the timer roller is at odds with the above-described requirement to reduce dropouts by having a simple loading path with minimal contact between the rollers and the tape and to have a small wrap angle around the several rollers.